A principal objective of coal benefication is to increase the calorific heating value or amount of thermal energy of the coal that can be released during a subsequent combustion process. One method of increasing the thermal energy released during combustion of coal is to decrease the amount of moisture by subjecting the coal to a drying process. It will be appreciated that moisture in coal has no heating value and, although not environmentally harmful, facilitates depletion because evaporation of the moisture consumes a portion of thermal energy released during combustion of coal.
Another known method of increasing the thermal energy released during combustion of coal is to decrease the amount of volatile matter within the coal, and thereby increase the relative amount of fixed carbon in the coal. The amount of volatile matter within coal may be decreased by subjecting the coal to a pyrolysis process. Pyrolysis of coal in an oxygen deficient atmosphere removes volatile matter, e.g. low boiling point organic compounds and some heavier organic compounds, by breaking chemical bonds during the heating process. Breaking chemical bonds within coal during the heating process increases the relative percentage of elemental carbon which provides most of the calorific heating value when coal is burned.
A by-product of the pyrolysis step is a stream of volatile gases. There are known methods for processing the volatile materials released from the pyrolysis step, to condense the volatile materials into liquids for fuel and other associated products. However, where the raw coal contains little oil in its initial state, producing significant amounts of oil and other associated products will be difficult. Therefore, it would be advantageous if there could be developed an improved coal upgrading process for low rank coal having an initial low oil content.